Filling Tanks - How Hot is Too Hot?

[realdiver7]

I have an Alkin W31 3.5 cfm compressor with manual drains, moisture monitor, carbon monoxide monitor and Nitrox stick for continuous blending. I just started pumping my own air this summer and really enjoy the convenience of doing it myself. The Alkin is a great compressor and I have no complaints about it. Continuous blending is easy once you get the hang of it, follow instructions and pay attention at all times.

When I fill my steel 77s, steel 100s and aluminum 80s, I notice that the tanks (cylinders) heat up very quickly regardless of whether I'm filling Nitrox or just air, especially as they get more and more full. I expect a certain amount of heat from them, but have been wondering how hot they are actually supposed to get when being filled at such a slow rate. I usually alternate between them, filling each one partially, and then returning to top them off, but that seems like a lot of trouble and fast work before the over pressure relief valve pops.

When I say hot, I mean the tanks get very hot, especially around 2,000 psi, but not so hot that I cannot touch them. I can hold them firmly with the palms of my hands and not feel like I'm getting burned, BUT they still seem to be much hotter than what I have been used to getting back from the dive shop.

Questions....how hot can they get without having expansion and contraction problems that could result in bad hydros over time, or without having some sort of catastrophic wall failure? Any ideas on other methods used to fill them like closing the whip valve partially to slow down the flow, or is this bad for the valve? How do you personally determine how hot is too hot for your tanks?

Edited to add: I fill my tanks directly from the compressor and do not use a cascade system as I normally fill only about 4 tanks each weekend. Tanks are filled dry; not in a tub of water.

Thanks!

 

[JBRES1]

How are you filling the tanks ? from a cascade or direct from the compressor ?
I too have the Alkin and fill from a cascade.
I fill them at a slow rate of speed, I go from 500 or 700 psi up to 2400 psig in about 4 minutes then to 3400 psig in another 3 or 4 minutes for my steel 100's. I then let them cool of and top off to 3500 to 3600 for final fill.
Jim Breslin

 

[jamiep3]

I believe the Navy Dive Manual recomends 300 psi per minute dry, no more than 600 in a water bath, but filling them in a bath is out of fashion now. Good rule of thumb is if you cannot touch them comfortably, you are filling too fast.

 

[realdiver7]

How are you filling the tanks ? from a cascade or direct from the compressor ?
I too have the Alkin and fill from a cascade.
I fill them at a slow rate of speed, I go from 500 or 700 psi up to 2400 psig in about 4 minutes then to 3400 psig in another 3 or 4 minutes for my steel 100's. I then let them cool of and top off to 3500 to 3600 for final fill.
Jim Breslin

I fill my tanks directly from the compressor. Do not have a cascade because I don't fill many tanks. When you say you fill them at a slow rate of speed, do you leave the fill whip valve wide open and just rotate tanks until they are full? If so, then that's how I fill mine. It looks like your initial fill rate from 500-700 to 2400 psi in 4 minutes is about 425-475 psi per minute, and then you're slowing down more after that, but my main question is how are you manually controlling the fill rate?

Thanks for the replies so far!

 

[realdiver7]

I believe the Navy Dive Manual recomends 300 psi per minute dry, no more than 600 in a water bath, but filling them in a bath is out of fashion now. Good rule of thumb is if you cannot touch them comfortably, you are filling too fast.

I'll make sure to note approximately how many psi per minute I'm filling the next time around. I believe it's well under 300 psi per minute, and probably between 100-200 psi per minute based on the output of the compressor and the time it takes to fill from 500 psi to full. All of my fills are dry.

Thanks!

 

[JBRES1]

If you are filling direct from the compressor, you are filling at a rate of about 20 minutes a tank, and thats much slower than I am.
I have a globe valve on the cascade and at the whip that I use to control the flow. Neither valve is put into the full open position when filling tanks, but I do have the tank valve in the full open position while filling.

Jim Breslin

 

[vshearer]

I believe the Navy Dive Manual recomends 300 psi per minute dry, no more than 600 in a water bath, but filling them in a bath is out of fashion now. Good rule of thumb is if you cannot touch them comfortably, you are filling too fast.

Ditto. 'Nuff said.

 

[rcontrera]

You gotta remember that when filling directly off a compressor, you are starting with MUCH hotter air than a shop that fills from a cascade. No matter how slow you go, hot air is still hot air.

 

[Doc Harry]

I dunno guys, sounds like something is wrong. I just can't figure out what.

I have an Alkin W31 electric, too, and even when I fill on a warm day (90 degrees F) my tanks barely get warm with a dry fill. Doesn't matter if I am filling AL19s, AL80s, or steel 130s, my tanks barely get warm.

Limiting the fill rate from the compressor just jacks up the upstream pressure and it not necessary with the Alkin W31. Even when the whips are wide open, the Alkin W31 fill rate is only something like 100-200 PSI per minute. That shouldn't cause the tanks to heat up like realdiver7 describes.

As far as fill pressure is concerned, the rated pressure of your tank is measured at 70 degrees F. If it is warmer than 70 then overfill your tanks. If it is colder than 70 degrees then underfill your tanks. I believe the conversion is 5 PSI per degree.

So an DOT AL80 is rated at 3,000 PSI at 70 degrees F. When I fill it at 60 degrees, I only pump it to 2,950 PSI. When I fill it at 80 degrees I pump it to 3,050 PSI.

 

[oxyhacker]

Unless someone pulled a fast on and sold you a compressor with considerably more output than you think you have, or you are routinely experiencing combustions within the tanks and not noticing it (highly unlikely), then there is no way your tanks are going to overheat enough to matter with using a 3.5 cfm compressor. Yes, they will get hotter than with a slow fill from a cascade, but no they will not get hot enough to matter, and the fact that they don't get too hot to hold reinforces that. You'd have to heat an alloy tank up to 250 F before it would be time to start worrying, and a steel to over double that.

Most of us with compressors just figure out a fudge factor for how much to overfill the tanks to get the the final, cooled, pressure we want, something which is completely acceptable under CFR something-or-other, as long as you don't get carried away.

If it bothers you, and you are frequently filling mulitiple tanks, one way around it is to add another whip, so you can fill two tanks at a time, so the tanks have more time to dissipate the theat as they fill. This is how many of the mid-sized portable compressors, that will not be used with a cascade, do it.